Farmer 2 has a 27 hp tractor + moldboard plow = 30 min. 
Farmer 3 has a 35 hp tractor + moldboard plow = 15 min. 
(2013 Farmer using a 154 hp tractor + chisel plow = 5 min.) 
Also, rather than resting your team, with a tractor you could plow all day and put your son on the tractor all night as described in the Ken Burns’ documentary The Dust Bowl. Doing this so quickly meant a farmer focused on one crop to sell at a profit to make a living.
Several factors came together in the early 20th century to create the perfect conditions for industrial agriculture to expand. Not only did Ford’s assembly line and John Deere’s tractors debut, but also nitrogen fertilizer was not available until this same time. Di-nitrogen gas makes up 78% of the air we breath, however this form is inert, chemically unavailable to life as a nutrient. In 1909, Fritz Haber discovered how to turn di-nitrogen and hydrogen gases from the air into ammonia. In 1913 Germany began producing ammonia on an industrial scale (lead by Carl Bosch, hence the name Haber-Bosch process), as a precursor for ammonium nitrate, which is used in munitions even today. After WWI, the nitrogen became available as fertilizer. This process is still used today to produce nitrogen fertilizer, in fact it uses ~1-2% of the global energy supply. 
These elements (industry, assembly line, agriculture, synthetic chemicals, natural resources) are brought together in Diego Rivera’s monumental Detroit Industry frescoes (a specific mural technique), which I visited for the first time this past weekend at the Detroit Institute of Art. The frescoes are a series of murals on all four walls of a large indoor courtyard with a glass ceiling. The east wall, which is the first you see when you enter depicts how humans and agriculture come from the earth and her resources. Below is one part of the east wall.
 From http://www.livinghistoryfarm.org/farminginthe20s/machines_08.htm
 From http://en.wikipedia.org/wiki/Haber_process